A battery converts the chemical energy of active materials into electrical energy by means of an electrochemical oxidation-reduction reaction. A battery includes an electrolyte, a cathode, and an anode. Water-activated metal batteries (such as Li—H2O, Na—H2O, Al—H2O, and Mg—H2O galvanic cells)1 oxidize a metal at the anode (negative electrode) and reduce water at a cathode (positive electrode). These systems in general could achieve much higher energy storage densities if there were a way to continuously extract dissolved oxygen from seawater, and to transfer this oxygen to the battery electrolyte to be used as an oxidant in the place of water. 1 “X-Y battery” is a battery in which species X is oxidized, and species Y is reduced during galvanic discharge. For example, a metal-dissolved oxygen battery is a battery in which the metal is oxidized and dissolved oxygen is reduced during galvanic discharge.
ReactiveTheoretical energy densityTheoretical energy densitymetalusing H2O as oxidizingusing dissolved O2 as oxidizinganodeagent (MJ/L)agent (MJ/L)Li2231Mg3350Al4884
As shown in the above table, seawater-activated Al—H2O power systems could offer nearly two times their presently-attainable energy density if they were able to reduce the O2 dissolved in seawater, rather than if the Al—H2O power systems reduced only the seawater itself.
Alternately, a system capable of transferring dissolved O2 from seawater into the electrolyte of a metal-air battery (such as Li—O2, Na—O2, Al—O2, Zn—O2 and Mg—O2 galvanic cells) could allow these batteries to function in ocean environments, whereas they are now restricted to operate only in environments with a ready supply of gaseous oxygen.
Prior batteries that oxidize reactive metals, and reduce the oxygen dissolved in seawater, have operated without self-contained electrolytes i.e. at least one of the components of the electrochemical cell, including at least one of the cathode, anode, and electrolyte are open to seawater and are not separated by any barrier to the surrounding environment. In some prior batteries, the electrochemical cell uses the ocean as the electrolyte. This configuration allows these batteries to reduce the O2 present in seawater at low rates. However, without a contained electrolyte, the batteries suffer from high internal resistances, and are prone to biofouling and calcareous deposits on their positive electrodes. Additionally, such battery systems must operate at very low voltages (often a single cell), as series combinations of cells for higher voltages will result in shunt losses between cells though the shared electrolyte.